Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. For example, in laminations they can provide printability, transparent or matte appearance, and/or slip properties. They can further be used to provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. They can also be used to provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
For some applications of BOPP films, it is desirable to have very high heat seal strength and/or very high hermetic seal strength. Some snack food packages can be subjected to high altitudes during transportation (known as “Over the Mountain” in some packaging parlance) whereupon the gas-filled packages can expand due to the lower ambient air pressure, stressing the sealed areas greatly, and consequently bursting. In addition, in many snack food packaging applications in Southeast Asia, packages can be subjected to much abuse during shipping and handling; couple this with the consumer perception that highly inflated bags (“pillow packs”) are viewed as a higher quality package, then the snack food package for this region often requires extremely strong seals to withstand high gas pressure inside the bag.
U.S. Pat. No. 5,888,648 describes a multi-layer film composite structure for providing hermetic seals in packages. The structure of the film includes a substrate layer and a sealant layer wherein the sealant layer itself includes two layers: an intermediate layer and a sealing layer wherein the intermediate layer is of a different composition than the sealing layer. A large variety of materials are contemplated for the intermediate layer such as LDPE (low density polyethylene), LLDPE (linear low density polyethylene), EPB (ethylene-propylene-butene) terpolymer, EP (ethylene-propylene) copolymer, plastomers, or blends thereof. Such a structure provides high heat seal strengths and hermetic seals, but the structure can be expensive due to the use of a thick intermediate layer (5-9 μm) in conjunction with a thinner sealant layer (ca. 1.5 μm) and also requires the use of an in-line extrusion coater in conjunction with 3-layer coextrusion of the film.
U.S. Pat. No. 5,376,437 describes a 3-layer film design with superior sealability and burst strength for packaging bags. The multi-layer film relies upon a “cushion” layer combined with the sealant layer, somewhat similar in function to the “intermediate layer” of U.S. Pat. No. 5,888,648. A key property for the cushion and sealant layer is the respective degree of surface orientation of each layer. The cushion layer should have a lower degree of surface orientation than the sealant layer by a specified amount. EPB terpolymer and EMMA (ethylene-methyl methacrylate-maleic anhydride terpolymer) are contemplated in the examples as suitable for the cushion layer and are also relatively thick (3-5 μm). The sealant layer is also relatively thick (>2 μm). Moreover, the use of Laser Raman microscopy to ensure correct surface orientation is a rather cumbersome and sophisticated technique to use for routine quality control.
U.S. Pat. No. 6,326,068 describes a 4-layer multi-layer film for hermetic sealing via use of a thick intermediate layer (3-15 μm) for providing compliance during sealing, and a thinner heat sealable layer (ca. 1 μm) for providing adhesivity. Random ethylene-propylene copolymers and ethylene-propylene-butene terpolymers are suitable for the intermediate compliance layer. However, such a film design requires 4-layer coextrusion capabilities and thick intermediate layers of expensive copolymers.
U.S. Pat. No. 5,817,412 describes a 3-layer coextruded film for low seal initiating temperature packaging films using 2 of the 3 coextruded layers for heat seal properties: an intermediate layer and a top sealant layer. The thicknesses of both these layers, however, are relatively thin compared to the other films (1.51 μm for intermediate layer; less than 0.4 μm for top sealant layer). The intermediate layer can be copolymers of ethylene, propylene, and/or butene or blends of such copolymers thereof. In particular, the example uses a non-metallocene-catalyzed propylene-butene copolymer Tafmer® XR110T as the intermediate layer. However, although this reference demonstrates low seal initiation and good processability, its ultimate heat seal strength and hermetic properties do not seem to be that impressive. The use of expensive copolymers such as Tafmer® XR110T as the sole component of the intermediate layer renders such a film design expensive.
U.S. Pat. No. 5,527,608 describes a 4-layer heat sealable film suitable for metallizing which exhibits high heat seal strength and hermeticity. The invention describes a “dual core” layer including a polypropylene layer and an ethylene-propylene block copolymer layer. The EP block copolymer layer is a very thick layer (8 μm), being nearly half the thickness of the overall substrate. A thick heat sealable layer (ca. 4 μm) is coextruded onto the EP block copolymer side and an optional fourth layer of HDPE (high density polyethylene) is coextruded on the polypropylene side as a metal adhesion skin layer. In essence, the EP block copolymer portion of the “dual core layer” acts like the intermediate or cushion layers mentioned in the other art references, and in this case, combined with a relatively thick heat seal layer, provides for high heat seal strengths and hermetic seals. However, this film can be expensive to produce based on the thick layers of EP block copolymer and heat sealant resin as well as possible 4-layer coextrusion equipment being required.
US Patent application 20020164470 describes a 2-layer hermetically sealable film for packages which includes a base layer with a softening additive and a coextruded heat sealable layer. The heat sealable layer includes a polypropylene copolymer with ethylene and/or butene. The base layer is polypropylene combined with a “softening additive” which can consist of ethylene-propylene copolymers, ethylene-propylene-butene terpolymers, and hydrocarbon resins, with cyclopentadiene-based hydrocarbon resin being preferred. This additive serves to make the base layer a “compliant” layer for heat sealability improvement. This film provides exceptional improvement in heat seal strength and hermeticity. However, a very thick sealant layer (3-15 μm) must be used in conjunction with the softening additive and it has been found by the inventor of the present invention that hydrocarbon resins and conventional propylene-based copolymers are not sufficient in enhancing seal strengths significantly especially with relatively thin sealant layers (e.g. ≦2 μm).